This last point may be linked to the fact that for large beads the modelisation of the solids using a continuous medium even with the help of granular theory becomes very untrustworthy. The number of particles that can be put in the pipe cross-section is too small to be treated with statistical methods. For instance, numerical methods with Eulerian–Eulerian formulation such as the one presented in Ref. [22] is unappropriate to our case and even give unpresentable results. Full Lagrangian methods would also be very expensive owing to the nevertheless large number of particles. We are now developing two alternate methods: one is based on Lagrangian dynamics for the solids, the forces being prescribed
with the help of a two-layer global model for the fluid. The idea is to detect the position of the compact bed and to compute the concentration in the upper layer, and then to apply some bedload sediment transport equations [19] for the momentum balance in the bed. The second is to develop a Navier–Stokes code that uses volume penalization [23] that seem very promising to cope with the solids.